It was first reported by Poulain et al. in 1975 that the ternary mixture ZrF.sub.4.BaF.sub.2.NaF formed a transparent glass when fused at 800.degree. C. instead of a new crystalline laser host material that was the object of their investigation. M. Poulain and J. Lucas, "Verres Fluores au Tetrafluorure de Zirconium. Properties Optiques d'un Verre Dope au Nd.sup.3+, "Mat. Res. Bull. 10, 243-246 (1975). Their discovery aroused immediate widespread interest because this previously unknown class of fluoride glasses represented the only practical, amorphous material with infrared transparency extending beyond 6 microns. This desirable optical attribute is a direct consequence of the glasses being completely nonoxide in composition and has been the principal reason for the extensive development over the last twelve years.
It was quickly realized that the extended infrared transparency of fluoride glasses could lead to optical fiber waveguides with losses on the order of 0.01-0.001 dB/km, considerably better than the 0.1 dB/km theoretical limit of silica fiber. The operating penalty would be the need to develop transmitters and receivers that operate at 2.5 micrometers rather than at the 1.3/1.55 micrometers communication channels of present-day fibers. Although many technological issues regarding the practicality of fluoride glasses remain to be solved, optical fiber waveguides have been made by several groups with losses near 1 dB/km. The residual loss which so far has prevented these waveguides from attaining intrinsic behavior is due to impurities in the batch starting materials and to scatter associated with glass processing. Without exception, and despite considerable effort to develop alternative compositions, all of the low loss optical work has been done on multicomponent glasses which closely resemble the original Poulain formulation. The principal differences have been the addition of the stabilizing agents AlF.sub.3 and LaF.sub.3. A typical composition in mole % is 54 ZrF.sub.4, 22.5 BaF.sub.2, 4.5 LaF.sub.3, 3.5 AlF.sub.3, 15.5 NaF. HfF.sub.4 can be partially substituted for ZrF.sub.4 in order to achieve the refractive index difference required for the core glass/cladding glass structure of optical waveguides.
A persistent and, as yet, not completely understood complication associated with ZrF.sub.4 glasses is their tendency to form opaque dark regions within the glass even when processed under rigorously controlled laboratory conditions. In a 1985 review of fluoride glass making techniques, M. G. Drexhage, "Preparation and Properties of High Optical Quality Bulk Fluoride Glasses," Abstracts 3rd Int. Sym. on Halide Glasses, June 24-28, 1985, Rennes, France, Drexhage remarked, "Specimens prepared under dry inert atmosphere (Ar or N.sub.2) while apparently vitreous, always exhibit a gray tint or black inclusions to some degree, and highlight scattering. This reduction phenomenon has been noted by many workers." This behavior was attributed to the loss of fluorine from the melt at 800.degree. C., effectively chemically reducing ZrF.sub.4.
The reduced species are known to be darkly colored solids. Recently, ZrF.sub.4 glasses were deliberately reduced by melting in a zirconium metal crucible to generate black regions. The darkening is attributed to reduced zirconium which is at least partially comprised of Zr.sup.3+, i.e., ZrF.sub.3.
The formation of reduced zirconium amounts to catastrophic failure of the glass for optical purposes; consequently it has been the universal practice of workers in the field to prepare ZrF.sub.4 (and HfF.sub.4) glasses under oxidizing conditions to circumvent the problem. Oxidation has been accomplished either by adding NH.sub.4 F.HF to the starting materials prior to melting or by exposing the melt to gaseous oxidants such as CCl.sub.4, NF.sub.3, SF.sub.6, CF.sub.4, and O.sub.2, or by doing both.
Above 400.degree. C., NH.sub.4 F.HF decomposes and provides a source of fluorine and HF. This decomposition has been used since the earliest years of fluoride glass making to insure that residual oxides in nominally fluorinated starting materials are fully converted to fluorides. It was even an early practice to use oxide starting materials for lack of commercially available pure fluorides (e.g., ZrO.sub.2, La.sub.2 O.sub.3, etc.) and fluorinate in situ with a large excess of NH.sub.4 F.HF to produce clear glass. However, the use of NH.sub.4 F.HF has several disadvantages. It introduces a source of impurities to the glass, especially transition metals, rare earths, and complex anions (sulphate, phosphate), which are difficult to overcome because of the large amounts of NH.sub.4 F.HF that are frequently used. It also causes considerable fuming of the melt as a result of its decomposition to gaseous products, and this in turn leads to loss of starting materials and contamination of the furnace. For these reasons, many workers have tried to abandon NH.sub.4 F.HF in favor of more elegant and convenient methods of controlling ZrF.sub.4 reduction. The method of choice has invariably been to introduce oxidizing agents into the atmosphere over the melt (dubbed reactive atmosphere processing or RAP) and was first applied to fluoride glasses by M. Robinson, R. C. Pastor, R. R. Turk, D. P. Devor, M. Braunstein, and R. Braunstein, "Infrared Transparent Glasses Derived from the Fluorides of Zr, Th and Ba," Mat. Res. Bull 15, 735-742 (1980), who originally developed the technique for fluoride single crystal growth. CCl.sub.4 was used as a RAP agent and was very effective in controlling both zirconium reduction and hydration of the glass by trace amounts of water, according to Reactions (1) and (2), EQU ZrF.sub.3 OH+1/2CCl.sub.4 .fwdarw.ZrF.sub.3 Cl+1/2CO.sub.2 +HCl(2)
Similarly, exposure of the melt to O.sub.2 will oxidize reduced zirconium, with formation of zirconyl fluoride as shown in Reaction (3), but unlike CCl.sub.4 will not dehydrate the melt. EQU 2ZrF.sub.3 +1/2O.sub.2 .fwdarw.ZrOF.sub.2 +ZrF.sub.4 ( 3)
Both oxidants have the disadvantage of introducing nonfluoride anions into the glass and both chloride and oxide impurities have been identified as causing glass instability leading to devitrification. In the latter respect, NF.sub.3 is a preferred RAP agent because it partially dissociates into atomic fluorine at high temperatures which effectively oxidizes the melt without introducing potential crystal nucleating sites. However, NF.sub.3 is a highly corrosive material at the typical melting temperature for fluoride glasses (850.degree. C.), and is quite destructive of furnace elements.
In addition to the complication of reduced zirconium species in fluoride glasses, it is well established that the presence of trace amounts of moisture, either in the melting atmosphere or absorbed onto the batch materials, can lead to glass instability. This is thought to occur through the formation of ZrOF.sub.2 at elevated temperatures by reaction (4): EQU ZrF.sub.4 +H.sub.2 O.fwdarw.ZrOF.sub.2 +2HF (4)
Because of this difficulty with water, several laboratories now process fluoride glasses entirely in inert atmospheres except for the NH.sub.4 F.HF and/or RAP steps.
It would clearly be an advance in the field to devise a method for controlling the oxidation state of ZrF.sub.4 /HfF.sub.4 containing glass melts without the necessity of using RAP and incurring its attendant problems. It would also be highly advantageous to do the melting under moisture free conditions. Recently, a new and improved glass fabrication procedure was developed by us which incorporates these features.